Canada warbler COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
The reasons for the decline of the Canada Warbler are not known. Indeed, changes in Canada Warbler habitat on both the wintering and breeding grounds have not been directly linked to changes in the population over time. Habitat loss and degradation on the wintering grounds are thought, however, to be important factors in the decline of the species (Lambert and Faccio 2005). Below, threats to the species on the wintering range and also potential threats to the species on the breeding range in Canada are discussed.
The Canada Warbler winters in areas where pressure from human development is high and deforestation intensive (Terborgh 1989; Robinson 1997; Conway 1999). The forests of the northern Andes (primarily in Colombia), the main wintering grounds of the Canada Warbler, are among the most threatened in the world (Davis et al. 1997). Indeed, in Colombia alone, 1.5 to 2.2 million acres/year of forest was cleared during the early 1990s (World Press Review 1993). Approximately 90% of all primary forest in the northern Andes, including 95% of the cloud rainforest has now been cleared (Henderson et al. 1991) for agriculture, fuel wood, cultivation of illegal drugs and non-selective herbicide spraying to eliminate these drug crops (Davis et al. 1997). Habitat loss is also associated with pipeline developments and road construction in the northern Andes (Davis et al. 1997; See Habitat Trends).
Although Canada Warblers exhibit a degree of adaptability to human disturbance, they are negatively impacted by changes in habitat that decrease the forest understory and also the forest canopy (Conway 1999).
Draining of swamp forests for agriculture and urban development, largely between 1950 and 1980, in the northeastern part of the species’ range (Tiner 1984; Miller 1999) is thought to have contributed to the decline of Canada Warbler populations in eastern North America (Conway 1999). Continued maturation of forest in previously cleared farmlands may also have contributed to habitat loss in the northeastern portions of the range (Conway 1999).
Habitat has also been lost in other parts of the breeding range. Boreal mixedwood forest has been cleared for agriculture in western Canada (Senate Subcommittee on the Boreal Forest 1999; Hobson et al. 2002). Forest also been cleared for industrial development (i.e., road and pipeline construction, drilling sites, etc.) in the oil and gas sector in northwestern Canada (Cooper et al. 1997; Senate Subcommittee on the Boreal Forest 1999; Hobson et al. 2002; South Peace Bird Atlas Society 2006; See Habitat Trends).
Forest harvesting and the various silvicultural practices that adversely affect the development of the shrub layer in managed forests may also decrease habitat for the species (Askins and Philbrick 1987; Cooper et al. 1997; Norton and Hannon 1997; Schieck et al. 2000; Tittler et al. 2001). The impact of these factors is likely to vary, however, as birds will breed in 10- to 20-year old regenerating clearcuts.
Grazing by forest ungulates, such as the White-tailed Deer, that reduces the shrub layer may reduce the quality of Canada Warbler habitat in localized regions (Conway 1999). A study in Massachusetts found that Canada Warbler abundance dropped as deer abundance increased (DeGraaf et al. 1991). Similarly, a study on the Kentucky Warbler, which has a similar ecology to the Canada Warbler, found that warblers shifted from sites with excessively high deer densities to areas with lower deer densities (McShea et al. 1995). Deer populations are increasing throughout their range (Russell et al. 2001), which increases the likelihood that Canada Warbler habitat will be affected by grazing.
Information on the effects of habitat fragmentation on the Canada Warbler are mixed. Some studies suggest that the species is sensitive to fragmentation (Askins and Philbrick 1987; Robbins et al. 1989; Litwin and Smith 1992; Hobson and Bayne 2000c). On the other hand, other studies suggest that the species is tolerant to habitat fragmentation that results from forest harvesting (Schmiegelow et al. 1997; Schmiegelow and Monkkonen 2002), possibly because of the high rate of forest regeneration around fragments.
The occurrence of Canada Warblers during the breeding seasons is negatively affected by the proximity and length of paved roads in forested landscapes (Miller 1999; R. Zimmerling unpubl. data). Road development may be a particular threat to Canada Warblers breeding in the boreal mixedwood forests of northern Alberta, where a significant increase in road development associated with industrial development is expected until 2030 (Schneider et al. 2003).
Decline in insect outbreak cycles
Although not generally recognized as a spruce budworm specialist (Conway 1999), Canada Warbler densities have been observed to increase during outbreaks (Crawford and Jennings 1989). A recent theoretical study also suggests that the decline in Canada Warbler populations over the last 30-40 years may be a response to a decline in spruce budworm outbreaks during the same period (D. Sleep unpubl. data).
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